Engine out cold-start emissions generated before light-off of an exhaust system emission control device (e.g., a catalytic converter) may contribute a large percentage of the total exhaust emissions. Various approaches may be used by engine control systems to expedite the attainment of the catalyst light-off temperature. For example, expensive electric catalyst heaters may be used to generate the heat. As another example, various combinations of spark timing retard, valve overlap, and increased fuel injection may be used to expedite catalyst warming.
In another approach shown by Shimoda in U.S. Pat. No. 7,331,170, a plasma generator is coupled to an emission control device, downstream of an oxidation catalyst and upstream of a diesel particulate filter. Electricity is discharged by the plasma generator into the exhaust flow to maintain the operating temperature of the particulate filter in a target operating region.
However, the inventors herein have recognized a potential issue with such a system. Since the electrodes of the plasma generator are themselves exposed to exhaust gas flow, soot and particulates get entrained and accumulated on the electrodes. This can cause leakage of current, making it difficult for a voltage to be applied across the electrodes of the plasma generator, and hindering the further generation of plasma. To address this issue, Shimoda requires fuel to be periodically added upstream of the oxidation catalyst. The resulting heat generated at the oxidation catalyst provides sufficient heat to burn off the soot accumulated on the plasma generator. However, the need to add fuel and periodically regenerate the plasma generator results in degraded fuel economy. In addition, while the generation of plasma addresses the temperature requirement of the filter, the temperature requirement of other emission control devices coupled in the exhaust may remain unmet. As a result, exhaust emissions may still be non-compliant. As another example, with the upstream addition of ionized air, the engine may need to run rich to maintain the exhaust catalyst at stoichiometry, thereby degrading fuel economy.
The inventors herein have recognized that cold-start emissions may be better addressed by converting the cold-start hydrocarbons using an ionized air stream while an emission control device is cold instead of (or in addition to) focusing on expediting light-off of the emission control device during the cold-start. The use of ionized air may provide lower overall emissions that can be implemented in multiple engine configurations, including engines operating with different fuels (e.g., gasoline or diesel) as well as different fuel injection types (e.g., port or direct injection) with minimal interference with existing engine cold-start controls. In addition, by addressing the exhaust soot using the ionized air, the need for particulate filters may be reduced.
Thus, in one example, cold-start engine emissions may be addressed by a method for an engine comprising: introducing ionized air downstream of an exhaust catalyst responsive to catalyst temperature. In this way, ionized air may be used to burn off cold-start emissions until an exhaust catalyst is activated.
As one example, during an engine cold-start, while an exhaust catalyst warms up, cold-start emissions may be oxidized as they are generated using ionized air. Ionized air may be flowed downstream of the exhaust catalyst so that exhaust emissions left untreated by the catalyst can be addressed using the ionized air. For example, ionized airflow may be delivered so that a threshold fraction of aircharge received downstream of the exhaust catalyst is provided as ionized air. The flow of ionized air may be accompanied by spark retard, at a less aggressive clip, to expedite catalyst heating. In addition, rich engine operation may be limited while ionized air is flowed so as to protect against component overheating. Ionized air may continue to be delivered until the exhaust catalyst is activated, after which time the ionized air flow may be terminated. In addition to adjusting the ionized airflow responsive to catalyst temperature, the ionized airflow may also be adjusted based on exhaust particulate matter load. For example, ionized air may be flowed during tip-ins, in anticipation of a rise in exhaust soot load.
In this way, cold start emissions can be addressed as they are generated without needing to expedite catalyst heating using aggressive spark retard or dedicated catalyst heaters. This allows exhaust emissions to be reduced without requiring precious metal loading on exhaust catalysts. This reduces catalyst costs and complexity. By reducing the need for aggressive spark retard during cold-starts, NVH issues associated with aggressive spark retard, such as intake rumble from a near wide open throttle during the spark retard, can be reduced, improving drive quality. By introducing the air downstream of the exhaust catalyst (e.g., an oxidation catalyst or a three-way catalyst), the catalyst can warm up near stoichiometry. This allows the ionized air to oxidize the residual hydrocarbons without generating NOx at the exhaust catalyst. In this way, the introduction of ionized air downstream of the exhaust catalyst allows emissions reduction benefits to be achieved during an engine cold-start. It will be appreciated that in alternate examples, the ionized air may be introduced upstream of the exhaust catalyst. In such an embodiment, in addition to emission reduction benefits, heating benefits may also be realized. Specifically, with upstream introduction of ionized air, to maintain the three-way catalyst at stoichiometry, the engine would need to run rich to balance the ionized air stream. While this may expedite catalyst heating, fuel economy may be affected.
In hybrid vehicles, the use of ionized air also can provide the opportunity to delay engine pull-ups to a colder catalyst temperature. By also using ionized air to reduce exhaust PM emissions during cold-starts, as well as other conditions where exhaust soot levels are elevated, particulate matter filter life can be extended. Overall, the impact on fuel economy is improved while reducing exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.